Control of corrosive conditions in condensers of a fixed bed catalytic cracking unit



March 7, 1950 v c. H. WHITACRE 2,499,435

CONTROL QF CORROSIVE CONDITIONS IN CONDENSERS OF A BED CATALYTICCRACKING UNIT Filed Nov. 23. 1946 NORMALLY GASEOUS HYDROCARB ONS PLUSSTEAM l\ -RECYCLE CONDENSATE WATER CONDENSER NORMALLY LIQUID PRODUCTS TOSTRIPPER z o HYDRO- q L; o: A CARBON J V GASES 5 GAS AND l- WATER 5SEPARATOR CRACKED WATE o PRODUCTS SEWQR INVENTOR. CHARLES A.WHITACRE HISATTORNEYS Patented 1950 I. I; t I I UNITED STATES PATENT OFFICE,

.CONTROL OF CORROSIVE CONDITIONS IN CONDENSERS OF A FIXED BED CATA-LYYTIC CRACKING UNIT Charles H. Whitacre, South Euclid, Ohio, assignorto-,-The Standard Oil-Company, 'Cleveland, Ohio, at corporationof OhioApplication November 23, 1946, Serial No. 711,824

4 Claims. (Cl. 19.6-52) 2 The present invention relates o an p o arecondensed out. These condensates are subprocess of crackinghydrooarbons'wherein use is sequently led to an il and water separatorin made of a fixed catalyt c bed that is Subjec ed order to remove thecondensed Water from said alternately to cracking and regeneratingcondiheavier hydrocarbons. tions, and more particularly to S p having 5It is with the problem of inhibiting corrosion effect of r n o-am nimumth corro ive of the condenser tubes, baffies and spacers that efi'ectsof the vaporousreaction'products' on the this invention is primarilyconcerned. Condensing equipment fordinarilyfiomployed- The salientobject of the present invention, h a D r therefore, is to preventcorrosion of both the steel The condensers in Such fi d b d catalyticparts and the copper alloy tubes of the condenser rackin uni q i klybecome corr ded du in unit to the end thatthe life of the condenserperation ofsuch un tshey are made of tubes, spacers baffles and shellwill be considerably bundles f tubes f a pp r all y, su h as increased,the expense of replacing said units antimony odmilaltylmotalla o pp8 ;Zand t n periodically will be effectively eliminated and the a ycontaining a t m y as a sta ili r), attendant interruptions in theoperation of the and spacers and baffles of steel around which the.fixed bed catalyst cracking unit will be brought to reaction productsflow. Steelordinarily-formsta a minimum.

protective oxide or gsulfiderfilmtat-pH levels above 1 1 analyses of tcondensate water '7 .0 but is subject to extensive corrosion at pI-I edfrom t 11 and water eparator has Shown levels below about pper alioysponthe that during the cracking part of the cycle of other hand, resistcorrosion at pI-Ixlevelsv below operation of a unit of this type, wheein the. but do not form o fi s to procracking part of the cycle is often minutes dutect them from corrosion by basic solutions. Surration andthe initial hydrocarbon Charge is a lorisihgly, both the Steel d e opp roy light gas oil, thepH of the Water varies through p and the CondenserShell as Well, ecom 2 a range of. from about 5.0 1308.7. It 'is believedSeverely corroded on thoroil S e during e 09+ that this variation in thepH value of the coneration of fixed bed catalytic cracking units. The dn te ater is due to the fact that at the corrosion-is, in fact, soextensive asto eventually be innin of the cycle, when the charge co eplug the condensers up completely 'or cause -exinto contact with acatalyst which has just been ooSSivo l a e d halt operation of theentire regenerated with air or some oxygen-containing unit. Thisinvolves not only considerable exgas, organic acids are formed by-areaction of 1361156 in replacing c r od d Cond nser "tube thehydrocarbons with the oxygen absorbed on bundles or entire condenserunits but has the the regenerating catalyst. These organic acids furtherdisadvantage of requiring "the cracking are ap arently formed insufficient 'quantlty to unit to be idle from time to time for'repairs.oil-set, during one portion of the cracking part of In yp fi bedcatalytic cracking units the cycle, any'basicity due to the formation ofthat are now used s ly in refineri s, an ammonia from nitrogen compoundsin the oil oil charge, usually together with'a certain amount har e,with the result that during Sa p of Steam, is Subjected to a Cracking ptio by tion of the cycle the pH of the cracked products passing saidcharge through the catalyst bed. 40 goes below 6.5. As the cracking partof the cycle The catalyst must be regenerated periodically by progressesand the oxygen absorbed by the catap si h h i xy en r an oxyg n-containlst is used up, the formation of these organic ing gas. As a result ithas become a more orless acids is believed to decrease so thateventually standard procedure to operate such units in opduring thelatter portion-of the cycle the pH of;

crating y l of y thirty minutes wherein ea the cracked products reachesa peak at a level su oyolo includes a p r od-of catalyst regeneraabove8.0. From that it is concluded thatthe tion and a period during whichthe a y t pcrsteel parts of the condenser are corroded in the forms itsfunction of accelerating the cracking of earlier portions of thecracking part of the cycle the y ocarbons introduced into the crackingand that the copper alloy tubes are corroded in chamber. the laterportions thereof. The situation is be- The cracked products. together Wia y steam lieved to be further aggravated by the fact that initiallyincluded with the charge, pass from the the eriodic lowering of the pI-Iinto the range cra Chamber to a fractionoting below 6.5 either preventsthe formation of a proequipped with conventional overhead condenserstective oxide or sulfidefilm. on the steel parts or wherein the heavierhydrocarbons-and:thesteam destroys any such films that may be formed. in

the later portions of the cycle when the pH rises above 7.0.

Attempts to remedy this situation have been frustrated heretoforebecause of the great number of variables involved. The characteristics,properties and composition of the initial hydrocarbons, the duration ofoperative cycles and, as a result, the frequency and degree offluctuation in the pH value of the vaporous reaction products, aresubject to such great changes that attempts periodically to counteractexcessive acidity or alkalinity of the vaporous reaction products byadding basic or acid substances, respectively, have been found to beentirely impracticable.

One line of attack on this problem that was suggested, for example, wasa proposal to dilute the vaporous reaction products with a neutral fluidsuch as fresh water. Calculations have shown, however, that such a stepwould not achieve the desired result. When the ratio of fresh water,contacted with the vaporous reaction products, to condensate water is2:1 in a cracking operation carried out in cycles of ten minutes each,for example, the effect obtained thereby may be calculated, by methodswell known in the art, to result in fluctuations of the pH from amaximum of over 8.0 to a minimum of about 5.5. These fluctuations in thepH are sti l too great to effectively inhibit, much less prevent, severecorrosion of the condenser parts. Attempts to further reduce thefluctuations in the pH of the vaporous reaction products by this meanswould involve the use of quantities of water that are prohibitive fromthe standpoint of economy of operation and that would necessarilyovertax the volumetric capacity of existing condenser units.

It has now been found that the water collected from the conventional oiland water separator, when recycled through the oil side of the condenser, has the effect of minimizing the pH fluctuations to a fargreater extent than would be expected on the basis of the aforementionedcalculations.

The accompanying drawing is a self-explanatory flow diagram illustratingthe method of this invention.

Trials conducted with a conventional fixed bed catalytic cracking unitoperating in cycles of thirty minutes each wherein the initialhydrocarbon charge is a light gas oil and the ratio of recycle waterintroduced into the oil side of the condenser to condensate watercondensed out from the reaction products is 2:1, revealed that the pH ofthe water separated from the oil and gas separator was reduced to amaximum of about 7.5 and a minimum of 6.8 and that no furtherappreciable corrosion of the condenser parts took place.

While the present invention is not to be limited in scope by any theoryherein advanced, it is believed that this action of the condensatewater, referred to hereinafter as recycle water, is probably due to thefact that it, being an accumulation of condensate from several operatingcycles, contains salts which act as buffers that serve effectively toreduce the maximum pH and raise the minimum pH of the products flowingthrough the oil side of the condenser.

The advantages of this method of operation are tremendous both from aneconomic and an operational point of View inasmuch as the recycle wateris readily available from the existing apparatus, no fresh water orbuffering solution need be supplied from an outside source and nostructural changes are required in the existing equipment. Furthermore,the equilibrium pH range in the condenser may be effectively raised orlowered with facility by the addition of an acid or base to the recyclesolution.

The amount of condensate water that can be recycled effectively inaccordance with the present invention is ordinarily dependent only uponequipment limitations and the amount of condensate water formed. Ingeneral it has been found that the ratio of recycle water to condensateWater should not be lower than about 1:1 and it may be as high as 10:1.In order to prevent the buiding up of excessive quantities of recyclewater, a portion of it is withdrawn from the system at a rate equal toabout the rate of formation of condensate water after a sufiicientquantity of recycle water has been collected.

Inasmuch as innumerable variations in this process may be made withoutdeparting from the principle of the invention, it is to be understoodthat such variations and modifications that come within the spirit ofthe invention herein disclosed are intended to be included within thescope of the following claims.

I claim:

1. In a process of cracking hydrocarbons which includes alternatelypassing said hydrocarbons over a catalyst and regenerating saidcatalyst, condensing at least a portion of the vapors produced bypassing said hydrocarbons over said catalyst, said vapors includingsteam and being excessively corrosive to the condensing apparatus, andseparating the condensate water from the condensed hydrocarbon products,the steps which comprise collecting said condensate water and contactingsaid vapors with from about one to about ten parts of said collectedcondensate water to one part of steam condensed from the reactionproducts.

2. In a process of cracking hydrocarbons which includes alternatelypassing said hydrocarbons over a catalyst and regenerating saidcatalyst, condensing at least a portion of the vapors produced bypassingsaid hydrocarbons over said catalyst, said vapors including steam andbeing excessively corrosive to the condensing apparatus, and separatingthe condensate water from the condensed hydrocarbon products, the stepswhich comprise collecting said condensate water and contacting saidvapors with about two parts of said collected condensate water to onepart of steam condensed from the reaction products.

3. In a process of cracking hydrocarbons which includes alternatelypassing said hydrocarbons over a catalyst and regenerating saidcatalyst,

condensing at least a portion of the vapors produced by passing saidhydrocarbons over said catalyst, said vapors including steam and beingexcessively corrosive to the condensing apparatus in fluctuating over awide pH range, and separating the condensate water from the condensedhydrocarbon products, the steps which comprise collecting saidcondensate water, selectively raising and lowering the equilibrium pHrange thereof to a level that is non-corrosive to the condensingapparatus, and contacting said vapors with from about one to about tenparts of said collected condensate water to one part of steam condensedfrom the reaction products.

4. In a process of cracking hydrocarbons which includes alternatelypassing said hydrocarbons over a catalyst and regenerating saidcatalyst, condensing at least a portion of the vapors produced bypassing said hydrocarbons over said 76 catalyst, said vapors includingsteam and being excessively corrosive to the condensing apparato onepart of steam condensed from the reaction products, and withdrawing saidcollected condensate water from the system at a rate approximately equalto the rate at which steam is condensed-.from the reaction products tomaintain the ratio of collected condensate water to condensed steamsubstantially constant and prevent 6 the building up of an excessivequantity of recycle water.

C. H. WHITACRE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,688,830 Pollock Oct. 23, 19282,400,431 Montgomery et a1. May 14, 1946 2,406,547 Kuhl et a1 Aug. 27,1946

1. IN A PROCESS OF CRACKING HYDROCARBONS WHICH INCLUDES ALTERNATELYPASSING SAID HYDROCARBONS OVER A CATALYST AND REGENERATING SAIDCATALYST, CONDENSING AT LEAST A PORTION OF THE VAPORS PRODUCED BYPASSING SAID HYDROCARBONS OVER SAID CATALYST, SAID VAPORS INCLUDINGSTEAM AND BEING EXCESSIVELY CORROSIVE TO THE CONDENSING APPARATUS, ANDSEPARATING THE CONDENSATE WATER FROM THE CONDENSED HYDROCARBON PRODUCTS,THE STEPS WHICH COMPRISE COLLECTING SAID CONDENSATE WATER AND CONTACTINGSAID VAPORS WITH FROM ABOUT ONE TO ABOUT TEN PARTS OF SAID COLLECTEDCONDENSATE WATER TO ONE PART OF STEAM CONDENSED FROM THE REACTIONPRODUCTS.